The instant invention was designed and developed to meet a need which exists to measure the biomass contained in the tree trunks or the like present in the world's great rain forests. The ability for any sensor or combination of sensors to measure carbon to an accuracy of 1 ton per hectare, or biomass to 2 tons per hectare, in an absolute method as compared with the existing ground truth is probably not possible. This is due to the currently available methods of determining ground truth. The state of the art in ground truth measurement accuracy is optimistically +/−10-20%. With this much expected variation in ground truth, the ability for any sensor to demonstrate an absolute accuracy of 1 ton per hectare is not possible relative to manually obtained ground truth.
Yet, the remote detection of the above ground component of forest biomass is one of the primary goals of scientists interested in Earth's carbon cycle. More recently, as a consequence of international environmental treaties, methods for accurately measuring biomass and carbon are being sought by both policy makers and commodity traders. The remote sensing community has responded to the challenge by taking diverse observational approaches including optical and multi-spectral imaging, laser altimetry, and synthetic aperture radar sensing. Because of the difficulty in penetrating high volume and structurally complex canopies commonly found in tropical rainforests using radars operating in the 440 to 10,000 MHz range, there is increasing interest in exploring the use of VHF radar (primarily 20 to 120 MHz) approaches for measuring biomass in heavy forest stands. VHF radars may also have potential for exploring the below ground component of biomass and possibly soil factors important in carbon sequestration and flux.
Previous experiments have shown that VHF frequencies have potential for measuring forest biomass in dense forests. The results from an in situ field experiment showed that frequencies below 120 MHz were required to achieve good discrimination between two forests of similar species composition having very different biomass densities (89 and 300+tons/ha of above-ground dry biomass respectively). Another study describes results from the 20-90 MHz CARABAS system where reasonably accurate biomass estimates could be made in forests ranging from 100 to 300 tons/ha. As of yet, no VHF signal saturation limit with respect to biomass has been identified.
To further explore the potential of VHF radar for biomass measurement, a VHF system was specifically designed to survey terrestrial biomass in forest stands above 100 tons/ha at a rate of 30 square miles per day. This system is disclosed in U.S. Pat. No. 5,886,662, to Johnson, the entire contents of which are incorporated herein by reference. The instrument was flown in the United States in 1997 and in Central America in 1998, and in three separate states during July 2003. The Central American deployment used the NASA C-130 aircraft for a series of flights designed to test several instruments for biomass retrieval and structural analysis in tropical forests. The sensor was flown over eleven different forest study sites within the protected watershed along the Panama Canal from the Atlantic coast to the Pacific. The instrument demonstrated a ˜+/−10% accuracy compared to ground truth as established by the Smithsonian Tropical Research Institute (STRI). Subsequent to this, the system according to the techniques disclosed in Johnson was installed in a Twin Otter and conducted flight surveys for the determination of carbon in terrestrial biomass in July 2003, and again in February 2005.
The basic BioSAR technology is described and defined in M. L. Imhoff, P. W. Johnson et al, “BioSAR: An Inexpensive Airborne VHF Multiband SAR System for Vegetation Biomass Measurement”, IEEE Transactions on Geoscience and Remote Sensing, Vol. 38(3). p. 1458-1462, May 2000, the contents of which are incorporated herein by reference.
Briefly, an algorithm was developed to estimate vegetative biomass from VHF radar data. The data used as an input to this model came from the Swedish FOA CARABAS SAR system, and was collected in a DARPA FOPEN experiment.
The data was in the form of SAR images, so each pixel represented the normalized radar cross section (NRCS) of a 3-meter by 3-meter square. This measurement is a weighted average of the NRCS response across a frequency range from 20 to 90 MHz. Additionally, the SAR image formation process requires integration across an angle given by: where ØINT is the integration angle,
      ϕ    INT    =            λ              2        ⁢                  δ          cr                      ⁢          λ      .      is the electromagnetic wavelength and δcr is the cross range resolution. For the CARABAS technique to achieve a 3-meter cross range resolution, the integration angle must be greater than 66 degrees. This has the effect of smearing both the incidence and azimuth angle responses.
This earlier experiment was originally designed to look at the issues associated with foliage penetration radar with military utility. Thus, the ground truth was not taken in a manner consistent with the standard techniques for biomass measurement. In particular, the ground truth data appears to represent the 10 closest trees, as opposed to the total number of trees in a given area. Tree density therefore had to be estimated.
A physics based, semi-empirical model was developed relating biomass to normalized radar cross section. This approach was chosen because the scattering geometry is complicated enough that an analytical model of reasonable accuracy is not tractable and, as of that time, there was insufficient data to construct an empirical model.
As the understanding of BioSAR has matured, this situation has changed, and empirical modeling is now based upon collected data and ground truth.
The resulting model relating biomass to normalized radar cross section is given by:Biomass=b1*(σ0−a110 log10(cos θ)+a210 log10(k)−a310 log10(|Rs|)+b2)+b3 where a1−a3 and b1−b3 are fit coefficients, σa is the Normalized Radar Cross Section (NRCS) measurement in dBm2/cell area dBm2, θ is the incidence angle of the specific angle bin developed by the Doppler processing, k is the electromagnetic wavenumber, and the surface reflection coefficient Rs is given by:
            R      s        =          Γⅇ              -                              (            Δϕ            )                    2                          Δϕ    =                  4        ⁢        πΔ        ⁢                                  ⁢        h        ⁢                                  ⁢                  sin          ⁡                      (            θ            )                              λ      where θ is the incidence angle, Δh is the rms surface roughness of the underlying soil and λ is the electromagnetic wavelength. These adjustments are made prior to incidence angle and frequency averaging.
The constants a1−a3 and b1 were originally derived from CARABAS data over a similar frequency range. These coefficients have been refined as a database of observed data was built. The wavelength, wavenumber, and the incidence angle are from BioSAR. Since soil moisture and surface roughness were not measured in situ, the surface reflection coefficient Rs was assumed to be 0.5. The Panama field-based biomass estimate for a single site was used to determine a system-related calibration offset (coefficient b2) as 3.5 dB. The coefficients b2 and b3 are used to calibrate for the observed dynamic range to allow for the variable soil moisture for the site being surveyed. The NRCS to biomass model used is an original equation. The NRCS for each pixel must be corrected for each frequency and for each look angle. The biomass model is then applied to all of the BioSAR signal data for the adjacent flight lines yielding biomass estimates for each 30 m by 300 m cell in the array.
The array is input to a geographic information system where a linear interpolation routine is applied to construct a 30×300 meter resolution biomass map of the transects across the survey area.
The NASA/Goddard Space Flight Center funded the construction and operation of the Portable Airborne Laser System (PALS), a small, compact, inexpensive LiDAR designed to sample extensive forest resources. As a proof-of-concept, the system was used to complete a multi-resource inventory of Delaware. Forest biomass, carbon, impervious surface area, open water area, and wildlife habitat estimates were derived from the airborne LiDAR measurements. Year 2000 PALS-based volume and biomass estimates were compared to 1999 US Forest Service—Forest Inventory and Analysis (FIA) county and state estimates. Statewide laser-based estimates of merchantable volume were within 5-15% of USFS-FIA state estimates; biomass estimates differed by 5-20%, depending on the model considered. The total above-ground dry biomass values were used to calculate a carbon budget for the state, by stratum within county.
The stability of the laser estimates as a function of sampling intensity were investigated by looking at the variability of county and state estimates using different combinations of laser flight lines. Empirical observations suggest that forest dry biomass and therefore, above-ground carbon, can be repeatedly estimated regionally (areas >5200 km2) within a range of ˜7 t/ha (˜3.5 t/ha C) with a systematic sample of flight lines spaced 6-8 km apart, e.g., a sampling intensity of 0.15 km/km2. The range of the biomass estimates will decrease as the size of the study area considered increases and as sampling intensity increases, e.g., as the spacing between systematic flight lines decreases. A study to discern the effects of increased sampling intensity, up to a sampling intensity of 1.0 km/km2 (equivalent to a one kilometer spacing between flight lines) is ongoing.
The PALS project was funded by NASA to investigate the sensor fusion of the PALS and Johnson's BioSAR data.
Johnson's BioSAR system, an airborne radar, and PALS, an airborne LiDAR, both specifically designed to measure forest biomass from low-altitude aircraft, were flown together aboard a Twin Otter over 30 selected sites in North Carolina. Collinear and coincident LiDAR and Radar data, along with video data were acquired. Refinements to the equations to predict above-ground biomass and carbon were developed separately for the LiDAR and Radar data sets to determine the accuracy, consistency, and variability of resultant biomass estimates.
It was expected that the combined LiDAR and Radar data can be used to estimate biomass more accurately than either instrument alone. Based upon this supposition, a multi-sensor platform and sampling scenario is offered in the instant invention which has the design goal of the detection of C changes of 1 t/ha over large areas. Such synergy is expected (though not yet documented) because the two instruments respond to different biophysical aspects of the forest target. The radar provides a volumetric return related to the total amount of large, woody material in the radar footprint. The laser, on the other hand, responds to changes in tree height and absolute above-ground level (AGL) of the aircraft. Expected improvements are, in fact realized by combining radar and laser data, then these improvements manifest themselves in the form of higher biomass estimation accuracy and decreased sampling (flight) costs.
Cost effective and accurate means to remotely determine aboveground standing biomass and the subsequent conversion to estimates of carbon stock are of critical need for both the scientific study of carbon cycle science and for commercial purposes in the private sector. The most viable technologies to obtain this data from aircraft or satellites are the active radar and LiDAR sensors. At present, there are no airborne or spaceborne radar systems capable of measuring terrestrial biomass in stands of over 100 tons/ha. Other systems do not address the direct measurement of biomass. The earlier Johnson system is the only US system available for civilian use that operates in the 100 MHz region and uses novel technology to mitigate electromagnetic interference and thus achieve NTIA and FCC approval. The instant invention provides modifications to the existing design of the original Johnson system for the fabrication of a new prototype with a design goal to measure increases of carbon fixation or sequestration by 1 tonne per hectare per year.
The instant invention could have a major impact on the current methods by which spatial information about forest biomass and forest carbon is obtained by both the public and private sectors. For example, current technologies utilize optical remote sensing techniques to determine vegetation cover types and information that may be locally-to-regionally conditioned by ancillary field observations. Also, field sampling strategies can be greatly enhanced and made much more accurate and cost effective. Both the public and private sector resource managers have a strong interest in reducing costs, improving efficiency, and increasing the timeliness and accuracy of geospatial information essential for decision making.
The development of larger-scale biomass estimation approaches ultimately depends on the accuracy of the smaller-scale technologies such as the approach disclosed herein. The technology disclosed herein also could lead to larger area coverage in regions where frequent biomass monitoring is required and currently not available from any airborne or spaceborne technique.
There also is significant commercial potential. The entire Forest Inventory Agency (FIA) operation could be expanded by orders of magnitude in scope with very little increase in the field work overhead. There currently is a real lack of forest structure and biomass measurement instruments and the instant invention will have considerable appeal in many markets.
Existing biomass and carbon survey methods and technologies are slow, expensive and inexact. The instant invention offers a new technology which provides for the airborne remote sensing of biomass and carbon using a system comprising VHF radar, a laser radar system, and a video camera. Thus, it will be appreciated that new and improved biomass and carbon measurement techniques are required that will provide for the validation of technologies developed to enhance net long-term carbon sequestration in above ground terrestrial biomass.
The existing techniques disclosed by Johnson have been designed and demonstrated to measure biomass and carbon over large areas at a rate of about 30 square miles a day with a demonstrated accuracy of +/−15%. Commercial biomass and carbon survey services are now being offered at a cost of about $2.00 per acre.
The instant invention is a variant of the original Johnson techniques combined with a new laser radar system and a new large linear array radar antenna. Thus, the instant invention allows for a much more exact survey with a design goal of 3 square miles (777 ha) per day. One design goal for the measurement of the change of carbon loading for the proposed system is 1 tonne of C per hectare per year. That is, the instant invention may measure the increase of carbon fixation or sequestration per hectare per year.
The instant invention puts forth the premise that the techniques disclosed herein may, for example, measure the change in biomass and carbon over the period of one year. The instant invention may, for example, provide an estimate of actual biomass and carbon.
A design goal of the instant invention is to be able to determine the change in carbon load at a rate of one ton per hectare per year. A better measure of the accuracy of the proposed system is the ability to obtain and achieve reproducible estimates of biomass and carbon over a short time course, and then observe reproducible results over the same stands after a period of one year which will demonstrate the expected change in biomass and carbon loads.
In one aspect of the invention, the planned addition and combination with a new Airborne Laser System will provide for two independent sensors. These sensors will be collocated on the same aircraft and will observe the same field of view. The two sensors will provide separate streams of data that will be used individually and collectively in a sensor fusion implementation.
In another aspect of the invention, the primary factors of the technical approach involve the addition of a laser radar system, the changes and refinements of the computer algorithms, and a new and large linear array antenna which will significantly reduce the radar footprint on the ground and allow for smaller and more accurate resolution cells or pixels.
Certain exemplary embodiments provide methods of remote measurement of terrestrial biomass contained in vegetative elements present in an area of interest. Such methods may comprise providing an airborne VHF radar system in combination with an airborne laser radar system. They may further comprise overflying the area of interest while directing radar energy from the VHF radar system toward the area of interest, wherein a plurality of radar resolution cells are defined on the area of interest. The VHF radar system can be used to collect backscatter data from the radar energy as a function of incidence angle and frequency for each of the plurality of given radar resolution cells in the area of interest. The laser radar system can be used to determine above ground level data and height of the vegetative elements in each of the plurality of radar resolution cells. A magnitude of the biomass in the vegetative elements can be determined based at least in part on the backscatter data and data from the laser radar system for each of the plurality of radar resolution cells in the area of interest.
Certain other exemplary embodiments provide a method of remote measurement of terrestrial biomass contained in vegetative elements present in an area of interest. Such methods may comprise providing an airborne VHF radar system in combination with a laser radar system, the VHF radar system comprising a linear antenna array. Such methods may further comprise overflying the area of interest while directing radar energy from the VHF radar system toward the area of interest, wherein a plurality of radar resolution cells are defined on the area of interest. The VHF radar system can be used to collect backscatter data from the radar energy as a function of incidence angle and frequency for each of the plurality of given radar resolution cells in the area of interest. The laser radar system can be used to determine above ground level data and height of the vegetative elements in each of the plurality of radar resolution cells. A magnitude of the biomass in the vegetative elements can be determined based at least in part on the backscatter data and data from the laser radar system for each of the plurality of radar resolution cells in the area of interest.
In certain non-limiting embodiments, the VHF radar system may be operably connected to an aircraft having two wings, with each wing having a wingtip, and the VHF radar system may extend from wingtip-to-wingtip. The VHF radar system may operate on a single frequency, or multiple frequencies (e.g. six frequencies). In certain non-limiting embodiments, the methods of may further comprise generating a map of the area of interest, the map showing at least the magnitude of the biomass of the vegetative elements as a function of location. Also, in some non-limiting embodiments, the methods may further comprise recording video and/or audio while overflying the area of interest.
In certain other exemplary embodiments, a system for remote measurement of terrestrial biomass contained in vegetative elements present in an area of interest is provided. Such systems may comprise a VHF radar system operable to be connected to an airborne object in a down-looking manner, the VHF radar system being further operable to direct radar energy toward the area of interest and to collect backscatter data as a function of incidence angle and frequency for each of a plurality of radar resolution cells defined on the area of interest. Such systems may include an airborne laser radar system operable to be connected to an airborne object in the down-looking manner, the airborne laser radar system being further operable to measure above ground level data and height of the vegetative elements in each of the plurality of radar resolution cells. Such systems also may include a processor operable to determine a magnitude of the biomass in the vegetative elements based at least in part on the backscatter data and data from the laser radar system for each of the plurality of radar resolution cells in the area of interest.
In yet other exemplary embodiments, a system for remote measurement of terrestrial biomass contained in vegetative elements present in an area of interest is provided. Such systems may comprise a VHF radar system operable to be connected to an airborne object in a down-looking manner, the VHF radar system being further operable to direct radar energy toward the area of interest and to collect backscatter data from the radar energy as a function of incidence angle and frequency for each of a plurality of radar resolution cells defined on the area of interest. Such systems may also include an airborne laser radar system operable to be connected to an airborne object in the down-looking manner, the airborne laser radar system being further operable to measure above ground level data and height of the vegetative elements in each of the plurality of radar resolution cells. Such systems may further comprise a processor operable to determine a magnitude of the biomass in the vegetative elements based at least in part on the backscatter data and data from the laser radar system for each of the plurality of radar resolution cells in the area of interest, wherein the VHF radar system comprises a linear antenna array.
In some non-limiting embodiments, the VHF radar system and the laser radar system are disposed so as to be collocated and collinear, and/or have the same field of view. In some non-limiting embodiments, the VHF radar system, the laser radar system, and/or the recorder are disposed so as to be collocated and collinear, and/or have the same field of view.